Variable selectivity circuits



April 1940' s. w. VAN INGEN SCHENAU 2,195,276

VARIABLE SELECTIVITY CIRCUITS Filed Oct. 15, 1937 INVENTOR BERNARDl/S l4. VININGEN SCHENAU BY #6 g ATTORN EY Patented Apr. 9, 1940 VARIABLE SELECTIVIT'Y omcurrs Bernardus W. van Ingen Schenau, Eindhoven, Netherlands, assignor, by mesne'assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application October 15, 1937, Serial No. 169,119 In Germany December 3, 1936 6 Claims.

This invention relates to a radio receiving arrangement with variable selectivity, and has for its object to provide meanswhich allow of controlling either manually or automatically the selectivity in an extremely simple manner.

According to the invention in a'radio receiving arrangement comprising one or more tuned .oscillatory circuits one or more conductors are provided which are inserted in at least one of the oscillatory circuits and whose resistance is strongly dependent on the temperature, so that this resistance varies under the influence of the received high, or intermediate, frequency oscillations and/or of a direct current or alternating current of any desired frequency supplied to the said conductors, and a variation of the damping of the oscillatory'circuits and/or of the mutual coupling of at least two of the .oscillatory circuits is brought about. The conductors whose resistance is strongly dependent on the temperature are preferably constituted by incandescent lamps of the kind used in electric pocket lamps. These incandescent lamps have a very small resistance in the cold state, whereas the resistance in the hot state is appreciably higher. With a 4 volts incandescent lamp of 160 mw. for instance, a resistance of about 10 ohms was measured in the cold state, whereas the resistance had a' value of about 100 ohms in the hot state. These incandescent lamps have theadvantages of a very low price and of a very small self-capacity, so that the filament may be heated by means of the received radio, or intermediate, frequency oscillations anda separate supply for selectivity-control can be dispensed with.

- The invention is not limited tothe use of pocket lamp bulbs, but includes also the use in the said way of other conductors whose resistance is strongly dependent on the temperature. It is also possible, for instance, to use conductors whose resistance decreases with increasing temperature. The invention will be more clearly understood by reference to the accompanying drawing rep resenting, by way of example, several modifications in Figs. 1 to 6 thereof.

The simplest form of construction of the invention is represented in Fig. 1 which shows an oscillatory circuit comprising an inductance coil 1L and a condenser C and forming part of the radio, or intermediate, frequency amplifier of a radio receiving circuit arrangement. An incandescent lamp G is connected in series with the condenser C. Since theincandescent lamp 55 Ghas agvery low self-capacity the radio, or intermediate, frequency current will traverse the filament of the incandescent'lamp. Upon receiving aweak signal the filament remains cold and the damping brought about by the incandescent lamp vmay be neglected. If, on the contrary, a strong 1 signal comes in the temperature of the filament will increase which involves an increase'of the resistance so thatthe oscillatory circuit will be clamped to a larger extent and the selectivity of the receiving arrangement will decrease. In this way automatic control of the selectivity in accordance with the amplitude of the incoming signal is assured so that the selectivity decreases with increasing amplitude of the received signal. Fig; 2 shows a band-pass filter which forms part of the radio, or intermediate, frequency amplifier of a radio receiving arrangement, and comprises two inductively coupled oscillatory circuits L1, C1 and L2, C2. Each circuit comprises an incandescent lamp G1 and G2 respectively connected in series with the tuning condenser. With an increase of the received signal the two oscillatory circuits are damped to a higher extent, which involves an increase of the width of the transmitted frequency band.

Fig. 3 represents a circuit arrangement in which With an increase of the amplitude of the received Y signal the current traversing the incandescent lamp will also increase. The resistance connected in parallel with the coupling condenser will con sequently increase due to which the coupling between the two circuits becomes closer, and the width of the transmitted frequency band is increased.

p In the above described arrangements the heating of the'filament was effected by the received radio, or intermediate, frequency oscillations. This method of heating generally requires, however, a comparatively strong current, so that the circuit including the incandescent lamp must be preceded by' one, or more, amplifying stages to secure perfect selectivity control. I For these ampliiying stages, no,nor only a very small, automatic gain control could be used, since in using a strong automatic gain control a substantially constant signal would be always supplied to the tube so that no selectivity control would be achieved. Moreover, the circuit arrangements referred to allow automatic control of the selectivity only in accordance with the intensity of the received signal, whereas a control in accordance with the intensity of one or more signals having an adjacent frequency is generally to be preferred. These disadvantages are avoided in the arrangements represented in'Figs. 4 and in which a separate control voltage is passed through to the incandescent lamps. This control voltage may be either a direct current voltage; or an alternating voltage having any desired frequency, for instance an alternating voltage derived from the power mains,'since due to the high inertia of the incandescent lamps there is no risk of modulation of the signal by the frequency of the control voltage. The latter may be varied either by hand, or automatically. Automatic selectivity control can be obtained by supplying to the incandescent lamps a direct current voltage depending on the intensity of the received signal, and/or on the intensity of one or more signals having an adjacent frequency. A direct influencing of the resistance of the in candescent lamps by the received signal can be avoided, if desired, by giving the control voltage a high value with respect to the amplitude of the signal occurring in the circuit, for instance, by effecting the selectivity control in an oscillatory circuit in which the received signal has still a small amplitude.

The circuit arrangement shown in Fig. 4 corresponds to that represented in Fig. 1, but with the difference that the incandescent lamp G is supplied with a separate control voltage E1- through the resistance R. In the same manner the arrangement shown in Fig. 5 corresponds to that illustrated in Fig. 3.

The circuit arrangements represented in Figs. 3 and 5, wherein the width of the frequency band transmitted by a band-pass filter is controlled by a modification of the coupling of the band-pass filter circuits, have the drawback that the middle of the transmitted frequency band is shifted by the control, and, moreover, the response curve of the band-pass filter is unsymmetrical. In fact the resonance frequencies of the band-pass filter are determined on the one hand by the natural frequency of the circuit L1 C1 C2 L2, and on the other hand by the natural frequency of the circuits L1 C1 C3 and L2 C1 C3. The effective capacity in the two last-mentioned circuits is altered upon controlling the band width, due to which one of the resonance frequencies is shifted, whereas the other resonance frequency remains unaltered. Moreover, the circuits L1 C1 C3 and L2 C1 C3 receive an addi tional damping due to which the resonance frequency determined by these circuits is damped to a higher extent than the other resonance frequency, so that the response curve becomes unsymmetrical.

These drawbacks are avoided by using the arrangement represented in Fig. 6. This figure shows a band-pass filter comprising two mutual coupled tuned circuits L1 C1 C4 and L2 C2 C5 in which an incandescent lamp G1 and G2 respectively is connected in parallel with the coupling condensers C4 and C5 respectively. The junction point of the condensers C1 and C4 is connected to the junction point of the condensers C2 and C5 through an inductance coil L3 which is connected in parallel with an incandescent lamp Ge. The control voltage Er is supplied to the series-connection of the incandescent lamps G1, G3 and G2 through the resistance R. The large condensers Cs and C7 serve to prevent a sort-circuit of the control voltage and a supply of control voltage to the amplifying tube succeeding the band-pass filter.

With an increase of the control voltage the resistance of 'the three incandescent lamps increases to the same extent. This involves a decrease of the effective capacity in the circuits L1 C1 C4 and L2 C2 C5, whereas the effective selfinductance in the circuit L1 C1 La C2 L2 increases. By giving the inductance L: a proper value the two resonance frequencies of the band-pass filters are shifted in an opposite direction by the same distance during the control operation, so that the middle of the transmitted frequency band remains unaltered. In this case the resistance of the incandescent lamp G3 is given such a value that the circuit L1 C1 L3 C2 L2 is damped to the same extent as the circuits L1 C1 C4 and L2 C2 C5 so that a symmetrical response curve is obtained.

What is claimed is:

1. In combination, a pair of tuned circuits, each circuit comprising an inductive reactance, a capacitive reactance and. a coupling reactance, a temperature-dependent resistance element connected across each coupling reactance, an auxiliary reactance of opposite sign to said coupling reactances operatively associated with the latter, a third temperature-dependent resistance element connected across the auxiliary reactance, and means for varying the temperature of said elements.

2. In combination, a pair of tuned circuits,

- each circuit comprising an inductive reactance, a

capacitive reactance and a coupling reactance, a temperature-dependent resistance element connected across each coupling reactance, an auxiliary reactance of opposite sign to said coupling reactances operatively associated with the latter, a third temperature-dependent resistance element connected across the auxiliary reactance, means for varying the temperature of said elements, and each of said resistance elements consisting of an incandescent lamp.

3. In combination, a pair of tuned circuits, each circuit comprising an inductive reactance, a capacitive reactance and a coupling reactance, a temperature-dependent resistance element connected across each coupling reactance, an auxiliary reactance of opposite sign to said coupling reactances operatively associated with the latter, a third temperature-dependent resistance element connected across the auxiliary reactance, said resistance elements each consisting of an incandescent lamp, and the filaments of the lamps being in series relation, and means for varying the temperature of said elements.

4. In combination, a pair of tuned circuits, each circuit comprising an inductive reactance, a capacitive reactance and a coupling reactance, a temperature-dependent resistance element connected across each coupling reactance, an auxiliary reactance of opposite sign to said coupling reactances operatively associated with the latter, a third temperature-dependent resistance element connected across the auxiliary reactance, means for varying the temperature of said elements, each of said coupling reactances being a condenser, and said resistance elements being in series with said varying means.

5. In a radio circuit, a tuned circuit including a coil in shunt with a condenser, an incandescent lamp having its filament in series-relation with said condenser and coil, means for varying the temperature of the filament with current derived from radio signals thereby to adjust the filament resistance, a second tuned circuit of the same frequency as the first tuned circuit, a coupling reactance between the two tuned circuits, and said filament being in shunt with the reactance.

6. In combination, in a band pass network, a pair of tuned circuits, each circuit including a coil, a tuning condenser and a coupling condenser, a temperature-dependent resistance element connected across each coupling condenser, an inductive reactance connected between one pair of like terminals of said resistance elements, a third temperature-dependent resistance element being across the reactance, and means for varying the temperature of said elements.

BERNARDUS W. VAN INGEN SCI-IENAU. 

